Rotary encoders (hereinafter also simply called encoders) are widely used as sensor devices for detecting the rotational angle of a rotationally driven member, such as power steering in an automobile, or the joint units of a robot apparatus. In the detection of a rotational angle using an encoder, various technologies have been proposed to further improve the detection accuracy.
For example, Patent Literature 1 discloses technology that converts the rotation of a primary rotary shaft into the rotations of two detection shafts by reduction gears with different reduction ratios, detects the rotational angle of each of the detection shafts with respective encoders, and computes an absolute angle of the primary rotary shaft based on the difference between the detected rotational angles, thereby accurately detecting the rotational angle of the primary rotary shaft. As another example, Patent Literature 2 discloses technology configured so that a relative angle encoder is disposed on the input shaft of a reduction gear joined to the rotary shaft of a motor, and an absolute angle encoder is disposed on the output shaft of the reduction gear. With this configuration, the relationship regarding the error that may be produced between the detection value of the relative angle encoder and the detection value of the absolute angle encoder is acquired in advance, and this relationship regarding the error is used to correct the detection result of the absolute angle encoder.